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Homework Help: Projectile: Find range, given two displacement points

  1. Mar 4, 2010 #1
    1. The problem statement, all variables and given/known data

    A particle is projected with initial velocity [tex]u\cos\alpha\vec{i}+u\sin\alpha\vec{j}[/tex] m/s from a point 0 on a horizontal plane.

    If this particle passes through two points whose displacements from 0 are [tex]3\vec{i}+\vec{j}[/tex] and [tex]\vec{i}+3\vec{j}[/tex]

    show that the range is [tex]\frac{13}{4}[/tex] and that [tex]\tan\alpha = \frac{13}{3} [/tex]

    2. Relevant equations



    S, displacement

    [tex]S = ut+\frac{1}{2}at^2[/tex]

    3. The attempt at a solution

    So, if the initial velocity is [tex]u\cos\alpha\vec{i}+u\sin\alpha\vec{j}[/tex] m/s, then the particle is projected at u m/s at angle [tex]\alpha[/tex] to the horizontal.


    [tex]S_x = ut\cos\alpha[/tex]
    [tex]S_y = ut\sin\alpha-\frac{gt^2}{2}[/tex]

    and if the displacement points are [tex]3\vec{i}+\vec{j}[/tex] and [tex]\vec{i}+3\vec{j}[/tex], then:

    [tex]S_x = ut_1\cos\alpha = 3[/tex]
    [tex]S_y = ut_1\sin\alpha-\frac{gt_1^2}{2} = 1[/tex]


    [tex]S_x = ut_2\cos\alpha = 1[/tex]
    [tex]S_y = ut_2\sin\alpha-\frac{gt_2^2}{2} = 3[/tex]

    I tried solving for the last two pair of equation. For each pair, I eliminated t. So I got two seperate equations in total involving [tex]\alpha[/tex] and u only.

    So here is where I'm stuck.
    I tried solving for [tex]\alpha[/tex] and u, but I just...can't!? I have no idea what I'm doing wrong, whether if it's algebraic error, or there's something wrong with the formulas. I've been stuck on this for the 5th hour now, and have redone this millions of times.. But still.. :confused:

    I'm beginning to suspect a problem with the question itself! Though I seriously doubt it.. :frown:

    Help'd be muchly appreciated!

    Thanks in advance..
  2. jcsd
  3. Mar 4, 2010 #2


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    Homework Helper

    You are on the right track. Show your pair of equation for u and tan(alpha) to check. I they are right it is easy to eliminate the term containing u and solving for the angle.

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